An X-ray CT device acquires X-ray transmission data of a subject, along with rotating a pair of an X-ray source and an X-ray detector opposed to each other, placing the subject therebetween, and performs calculations to reconstruct a tomographic image (CT image) therefrom. Such X-ray CT device may serve as an industrial and security-use survey instrument, a medical diagnostic imaging device, and the like.
A PCCT device equipped with a photon counting mode is one of the medical X-ray CT devices. In the PCCT device, a photon counting type detector counts photons of X-rays (X-ray photons) that have passed through a subject, with respect to each detector element. This configuration allows, for example, acquisition of a spectrum that enables estimation of elements constituting internal tissue of the subject, through which X-rays have passed. Accordingly, it is possible to obtain an X-ray CT image on which element-level differences are depicted in detail.
The PCCT device categorizes individual X-ray photons being counted, according to energy values, and thus X-ray intensity can be obtained on the energy value basis. By utilizing this feature, the PCCT device may extract X-rays only within a specific energy range, for reconstructing an image to be used in diagnosis. In this case, the X-rays falling outside the energy range are attenuated to a minimum, thereby reducing an exposure dose to a patient being the subject.
A method of attenuating the X-rays outside the energy range includes, for example, a technique of inserting between the X-ray source and the subject, a thickness-variable X-ray attenuating body (hereinafter, referred to as “X-ray filter”) (see Patent Document 1, for instance). The method of the Patent Document 1 allows reduction of the X-rays falling in an unnecessary energy range, by the use of the X-ray filter.